Common industrial methods for producing styrene typically include separation and purification processes such as distillation to remove unwanted impurities. Unfortunately, purification processes carried out at elevated temperatures result in an increased rate of undesired polymerization. Distillation is generally carried out under vacuum to minimize loss of monomer. The presence of oxygen, although virtually excluded in styrene distillation, will also promote polymerization of the monomer.
This polymerization results not only in loss of desired monomer end-product, but also in the loss of production efficiency caused by polymer formation and/or agglomeration of polymer on process equipment. Thermal polymerization, which typically occurs during distillation, of styrene monomer results in the formation of normal (i.e., linear) polymer. This resulting polystyrene polymer is characterized by its glassy and transparent appearance and its solubility in the styrene monomer and many organic solvents.
The compounds generally used commercially to inhibit polymerization of vinyl aromatic monomers are of the dinitrophenol family. For example, U.S. Pat. No. 4,105,506, Watson et al. teaches the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds. U.S. Pat. No. 4,466,905, Butler et al. teaches that a combination of 2,6-dinitro-p-cresol and p-phenylenediamines will inhibit polymerization in a distillation column when oxygen is present. U.S. Pat. No. 4,774,374, Abruscato et al. teaches compositions for inhibiting the polymerization of vinyl aromatic compounds. The composition employs an oxygenated product of the reaction of an N-aryl-N'-alkyl-p-phenylenediamine with oxygen. U.S. Pat. No. 4,720,566, Martin teaches the use of a hydroxylamine compound and a phenyl-p-phenylenediamine compound to inhibit the polymerization of acrylonitrile in a quench tower.
U.S. Pat. No. 3,674,651, Otsuki et al. teaches the use of benzoquinone for inhibiting the polymerization of acrylic acid. U.S. Pat. No. 5,396,004, Arhancet et al. teaches the use of a combination of a phenylenediamine compound and a hydroxylamine compound, preferably a hydroxyalkylhydroxylamine, to inhibit the polymerization of vinyl aromatic compounds during processing.
Czechoslovakia Patent No. 163,428 teaches a method for stabilizing styrene and divinylbenzene utilizing 2,4-dinitroorthocresol and diethylhydroxylamine. European Patent Application 240 297 teaches the use of this combination to inhibit polymerization of styrene. The use of diethylhydroxylamine however is problematic in styrene purification processes as it has a boiling point of 125.degree. C. to 130.degree. C. at 760 mm Hg that is similar to styrene and can be carried over with the styrene during processing.
A variety of inhibitor compositions have been employed in styrene and other vinyl aromatic monomers to inhibit undesired polymerization. Agents that have been used include sulfur, p-benzoquinone, phenylenediamines, tert-butyl pyrocatechol, phenothiazine, hydroxylamines and hindered phenols. However, many of these compounds present disadvantages such as high toxicity, instability, explosive hazard at elevated temperature and insufficient efficacy under processing conditions (i.e., inhibitor requires oxygen to be effective). The present inventors have discovered a novel composition which acts synergistically to inhibit vinyl aromatic monomer polymerization while avoiding these problems associated with the known inhibitors.